Spreading the Risk of Extinction By Genetic Diversity in Populations of the Carabid Beetle Pterostichus Oblongopunctatus F. (Coleoptera, Carabidae)

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Identification of insect species often is only possible with the help of characters that do not have a direct selective value. Such characters may reflect pleiotropic action of genes. This hypothesis was tested in the carabid Pterostichus oblongopunctatus. The closely related species P. quadrifoveolatus has 3 large pits on each elytron, and prefers sites where wood has been burned. P. oblongopunctatus is a common forest species and two groups of morphs can be recognized: low-pitters with 4-5 pits on each elytron in a somewhat winding row, and high-pitters with 6-8 (or more) pits in a straight row. In dry forests the proportion of high-pitters is lower than in moist forests. Assuming pit numbers are genetically determined this suggests that low-pitters are favoured by relatively dry conditions, and high-pitters by moist conditions. Moreover, the shift of the proportion of high-pitters from year to year is correlated with the amount of precipitation in May-August, the period of larval development, especially with the accumulated deviations from the normal amount of rain. The latter suggests a genetical base for the two groups of morphs, which was supported by comparing the progeny of low-pitters with that of high-pitters. Selection experiments suggest that low-pitters are favoured by dry conditions and high-pitters by moist conditions during larval development. Samplings from different parts of Europe were in accordance with the hypothesis: In Poland, with a lower precipitation than Drenthe (The Netherlands), the proportion of low-pitters was higher, and in the Bavarian Alps, with about the same mean amount of precipitation as Drenthe, but on heavy loam, the proportion of high-pitters was higher than in Drenthe. The occurrence of some genotypes with a different moisture tolerance in the same population allows the species to occupy a wide range of forests, and to live with relatively small fluctuations in numbers in areas with highly unpredictable rainfall. The chance of the population to go extinct is decreased by spread of the risk over some genotypes with a differing tolerance of moisture conditions. P. quadrifoveolatus (with only 3 pits and considered to be a good species) represents the morph with a high tolerance of dry conditions, viz. after a forest fire.